What is a ekg and how does it work. what are some conditions it can aid in diagnosing

Electrocardiogram: A recording of the electrical activity of the heart. Abbreviated ECG and EKG. An ECG is a simple, noninvasive procedure. Electrodes are placed on the skin of the chest and connected in a specific order to a machine that, when turned on, measures electrical activity all over the heart.

An EKG can help diagnose:
Abnormally fast or irregular heart rhythms.

Abnormally slow heart rhythms.

Abnormal conduction of cardiac impulses, which may suggest underlying cardiac or metabolic disorders.

Evidence of the occurrence of a prior heart attack (myocardial infarction).

An ECG or EKG is a tracing from a electrocardiograph that measures the electrical activity from the heart. Electrodes are placed on the arms and legs on the chest . There are three clearly recognizable waves. P-wave, QRS complex and T-wave. These components can be correlated with the electrical activity of the atrial and ventricular muscle. 
In summary is is a graphical representation of the sum of the electrical activity produced by all the heart muscle fibres as they depolarize and repolarize.
Most commonly used on conditions including hypertension and myocardial infarction.